Tunable THz Filter Research Articles

Investigation of resonant properties of metamaterial THz filters fabricated from vanadium dioxide thin films

Vanadium dioxide (VO2) is a promising candidate for electronic and optical switching applications in the terahertz frequency range due to the metal to insulator transition (MIT). In this study, the use of VO2 patterned as a metamaterial surface or coupled as a homogeneous layer with a metallic metamaterial surface on top is investigated in terms of performance. High-quality VO2 thin films were deposited on c-cut sapphire substrates by using the dc magnetron sputtering technique. A change in resistivity by a factor of 104 MIT in VO2 was observed allowing to investigate its use as a controllable metamaterial. The layer was patterned using a unique geometry (four-cross shaped) that operates in the THz frequency range. To understand its performance as a tunable THz filter, the four-cross structure fabricated from VO2 is compared to one fabricated from Au on VO2 bare film using UV lithography and ion beam etching techniques. The spectral performances of metamaterials were assessed using THz-Time Domain Spectroscopy (THz-TDS) and results were compared with simulations based on CST Microwave Studio. Absence of the resonant effects in the purely developed VO2 device, while clear observation of the MIT behavior shows the strong dependency of the inductive and/or capacitive effects of the four-cross structure on conductivity of the surface metamaterial, which is clearly observable for the Au-based device. In the latter case, the resonant transmittance of the filter can be effectively modulated by change in temperature.

Isolation control for implementing the single dielectric resonator based tunable THz MIMO antenna and filter

Isolation control for implementing the single dielectric resonator based tunable THz MIMO antenna and filter

Intensity-tunable terahertz bandpass filters based on liquid crystal integrated metamaterials.

In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry-Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

Actively tunable THz filter based on an electromagnetically induced transparency analog hybridized with a MEMS metamaterial

Electromagnetically induced transparency (EIT) analogs in classical oscillator systems have been investigated due to their potential in optical applications such as nonlinear devices and the slow-light field. Metamaterials are good candidates that utilize EIT-like effects to regulate optical light. Here, an actively reconfigurable EIT metamaterial for controlling THz waves, which consists of a movable bar and a fixed wire pair, is numerically and experimentally proposed. By changing the distance between the bar and wire pair through microelectromechanical system (MEMS) technology, the metamaterial can controllably regulate the EIT behavior to manipulate the waves around 1.832 THz, serving as a dynamic filter. A high transmittance modulation rate of 38.8% is obtained by applying a drive voltage to the MEMS actuator. The dispersion properties and polarization of the metamaterial are also investigated. Since this filter is readily miniaturized and integrated by taking advantage of MEMS, it is expected to significantly promote the development of THz-related practical applications such as THz biological detection and THz communications.

Metal-graphene hybridized plasmon induced transparency in the terahertz frequencies.

In this work, metal-graphene hybridized plasmon induced transparency (PIT) is systematically studied in the proposed simple metal/dielectric/graphene system. The PIT effect is the result of the coupling between the bright dipolar modes excited in the graphene regions under the shorter metallic bars and the dark quadrupolar modes excited in the graphene regions under the longer metallic bars. The coupled Lorentz oscillator model is used to help explain the physical origin of the PIT effect. Other than being tuned by the distance and the lateral displacement of the orthogonal metallic bars, the coupling efficiency can be further enhanced by the in-phase coupling or quenched by the out-of-phase coupling between the adjacent unit cells. Reduced barrier thickness will result in the enhancement of the coupling strengths and the scaling down of the device. Finally, we show that the PIT window can be actively tuned by changing the Fermi energy of graphene. The proposed structure has potential applications in actively tunable THz modulators, sensors and filters.

Ultrabroad and Angle Tunable THz Filter Based on Multiplexed Metallic Bar Resonators

An ultrabroad and angle tunable terahertz filter based on multiplexed metallic bar resonators is designed, fabricated, and characterized in this letter. We found that the bandwidth of the filter could be greatly extended by the multiplexed configurations. When the number of gold bars in a unit cell increases from 1 to 5, the bandwidth will be broadened from 0.13 to 1.20 THz. Besides, the modulation depth of the filter can be tuned by rotating the sample to change the angle between the long axis of the metallic bars and THz polarization. When the deflection angle decreases from 90° to 0°, the transmission of the filter decreases from 1 to 0.26. Therefore, the multiplexed metallic bar structure provides a feasible method to pursue a tunable broadband terahertz metamaterial filter.

Tunable Terahertz Hybrid Metal–Graphene Plasmons

We report here a new type of plasmon resonance that occurs when graphene is connected to a metal. These new plasmon modes offer the potential to incorporate a tunable plasmonic channel into a device with electrical contacts, a critical step toward practical graphene terahertz optoelectronics. Through theory and experiments, we demonstrate, for example, anomalously high resonant absorption or transmission when subwavelength graphene-filled apertures are introduced into an otherwise conductive layer. These tunable plasmon resonances are essential yet missing ingredients needed for terahertz filters, oscillators, detectors, and modulators.

Tunable THz perfect absorber using graphene-based metamaterials

A tunable THz absorber, with absorbance more than 90% is proposed, and numerically characterized. The absorber structure is based on metamaterials with unit cells consisting of two patterned graphene layers separated by a 5-nm thick layer of Al2O3. Numerical results show that when the chemical potential of the top graphene microribbons are tuned by an external variable bias and that of the lower graphene fishnet is kept at µC=0, frequency of the absorption peaks can be tuned as desired, therefore we can have a tunable or switchable absorber. The proposed absorber can have applications in designing tunable reflective THz filters or tunable THz switches and modulators. It can also be used for cloaking objects in THz range.

Tunable Fano resonance in hybrid graphene-metal gratings

Hybrid graphene-metal gratings with tunable Fano resonance are proposed and theoretically investigated in THz band. The grating contains alternately aligned metal and graphene stripes, which could be viewed as the superposition of two kinds of gratings with the same period. Due to different material properties, the resonance coupling between the metal and graphene parts forms typical Fano-type transmitting spectra. The related physical mechanism is studied by inspecting the induced dipole moment and local surface charge distributions at different wavelengths. Both of the resonance amplitude and frequency of the structure thus are adjustable by tuning graphene's Fermi energy and the grating's geometrical parameters. Furthermore, the Fano-type spectra are also quite sensitive to environmental indices, which supply another kind of tunability. All these features should have promising applications in tunable THz filters, switches, and modulators.

Development of stress-induced curved actuators for a tunable THz filter based on double split-ring resonators

Using stress-induced curved cantilevers to form double split-ring resonator (DSRR) in three-dimensional configuration, an electrically tunable microelectromechanical system (MEMS) based out-of-plane metamaterials THz filter is experimentally demonstrated and characterized. While the achieved tunable range for the resonant frequency is 0.5 THz at 20 V bias, quality factor of the resonant frequency is improved as well. This MEMS based THz filter using released DSRR structures shows its potential in tunable metamaterials applications such as sensors, optical switches, and filters.

Continuous Tera-Hertz wave transmission spectroscopy of Nb double superconducting split-ring resonator array

Transmission spectroscopy of two Nb double superconducting split-ring samples with different thicknesses on MgO substrates was measured by a continuous Tera-Hertz spectrometer. The transmission curves of two different samples with the thicknesses of 50 and 150 nm at 7.5 K show dips at 480, 545 GHz, respectively, which origin from the different capacities and inductances of the samples. For the sample of 50 nm, the dip shifts to lower frequency, also decreases in depth and increases in width with temperature or field increasing below Tc of Nb film, while the sample of 150 nm does not show such a phenomenon. This thickness-dependent transmission behavior is due to the kinetic inductance and conductivity change of superfluid electrons in Nb film and may suggest a practical tunable THz filter based on the thinner samples.

Thermally tunable THz filter made of semiconductors

By using the finite-difference time domain method, the transmission properties of terahertz wave passing through two different periodic subwavelength structures on InSb semiconductor slabs under different temperatures have been investigated. The results show that the peak transmittance increases and the transmittance peak has a blue shift when the temperature is increasing. It is extremely versatile to control the tunable THz filter by varying the temperature.

Control of enhanced THz transmission through metallic hole arrays using nematic liquid crystal

We demonstrate frequency tuning of enhanced THz radiation transmitted through a two-dimensional metallic hole array (2D-MHA) by controlling the index of refraction of the medium filling the holes and adjacent to the 2D-MHA on one side. The medium is a nematic liquid crystal (NLC) and its index of refraction is varied using magnetically controlled birefringence of the NLC. With the NLC, the peak transmission frequency of the 2D-MHA shift to the red by 0.112 THz and can be tuned from 0.193 to 0.188 THz. The peak transmittance is as high as 70% or an enhancement of 2.42 times, considering the porosity of the 2D-MHA. As a tunable THz filter, this device exhibits a continuous tuning range of 4.7 GHz , a low insertion loss of 2.35 to 1.55 dB and a quality factor of ~ 4-5.